This invention relates to barium titanate powders, and particularly to such powders having high density and purity, uniformity of particle size, and controlled stoichiometry. The invention also relates to a method of producing such powders by reacting barium hydroxide with titania or a titanium alkoxide in a protic solvent.
Finely divided powders of barium titanate are useful in the production of dielectric components. Specifically, many dielectric components are prepared from such materials which, when sintered, form a hard and durable insulating element. Particularly useful for this purpose is barium titanate which is in particulate form of relatively uniform particle size of about 1000 angstroms (0.1 micron). The material is also desirably of high purity, high density, and closely controlled oxide composition. Barium titanate materials having these characteristics, especially those related to particle size and density, tend to sinter at lower temperatures, saving time and energy in the production of ceramic articles based thereon, and in the specific case of layered capacitors, providing greater strength per unit thickness of the layer and increased resistance to dielectric breakdown. Uniformity of particle size is also desirable to prevent localized exaggerated grain growth, which can result during sintering of the layers of a capacitor when the particle size distribution is not narrow. A typically large grains which can grow as a result can cause flaws that adversely effect strength and dielectric properties.
Producing barium titanate particles that have pre-calcining density approaching the theoretical density is also desirable, both in terms of production cost and product quality. In the production of ceramic articles, the oxide powders are normally molded or pressed into a desired shape, the so-called "green" shape, or are tape cast, followed by sintering at elevated temperatures to fuse the powders into a coherent and strong body. Heretofore, barium titanate powders having a density of only about 5.5 g/cc or less have been produced in chemical processes. If low-density particles such as these are pressed into a shape and sintered, excessive shrinkage of the material can occur, which can reduce the strength and increase the likelihood of cracking. Although the powders can be calcined after (or during) their initial production in order to shrink and densify them prior to the sintering step, the additional time and energy required for calcining undesirably adds to the production cost. Furthermore, calcining of powders often causes agglomeration of the primary particles into larger units of about 1 micron or greater. Although these agglomerates can be pulverized to smaller sizes, the pulverized particles are often irregular in shape. Such irregular-shaped particles tend to leave larger void spaces when packed or pressed in the green state, which can weaken the final ceramic body, increase tendency towards dielectric breakdown, and reduce the capacitance.
Barium titanate and other crystalline multi-oxides having the desired properties discussed above have heretofore been difficult to attain. Barium titanate, for example, has been prepared by reacting titanium isopropoxide and barium hydroxide in KOH solution at a ph of 11-14. S.S. Flaschen, J. Amer. Chem. Soc., 77:6194 (1955). The cited reference does not define, however, the stoichiometry of the product (that is, the exact BaO/TiO ratio) or the purity of the product, in terms of whether the KOH can be successfully removed. In other procedures using titanium alkoxides and barium hydroxide, it has been difficult to control the BaO/TiO.sub.2 molar ratio close to the desired unity value. See, for example, K. Kiss et al., J. Amer. Cer. Soc., Vol. 49 (6), 291-302 (1966).
BaTiO has also been made by reacting alkoxides of barium and titanium, but this procedure has resulted in the formation of BaCO.sub.3 as an impurity within the crystal structure. K.S. Mazdiyasni, et al., J. Amer. Cer. Soc., Vol. 52 (10), (1969). It has been taught that the use of alkoxides results in the generation of BaTiO.sub.3 in very fine crystallite form. K.S. Mazdiyasni, et al., J. Amer. Cer. Soc., 55 (12) 633 (1972). However, the generation of barium alkoxide from barium metal itself, required by this process, is expensive and difficult. In other procedures directed to forming barium titanate, BaCO.sub.3 and TiO.sub.2 have been calcined together, in solid state, to generate BaTiO.sub.3 in situ. This procedure, however, can leave impurities in the crystalline structure, and although the BaTiO.sub.3 formed by the procedure is of high density, the grain size itself can be undesirably coarse. Further, the use of a high temperature calcining step, as reviewed earlier, is itself undesirable.
In none of these methods are highly dense, sub-micron particles formed without a calcination step or other high-temperature treatment. Accordingly, there remains a need for a method of economically producing barium titanate having the chemical purity and physical form, including density and particle size, required for the production of high quality ceramic materials such as dielectric components.